Optical duobinary transmission is a well-known modulation format in fiber optic communications. The duobinary transmission format is potentially a cost effective commercial fiber optic data transport solution, particularly for metropolitan applications. Characteristics that help make duobinary transmission potentially cost effective include a high tolerance to accumulated and/or residual dispersion, flexible requirements with respect to the placement of dispersion compensation units within a transmission system, a high tolerance to nonlinear penalty, and relatively low bandwidth requirements for the optical transmitter.
The optical duobinary transmission format transmits binary data using three states, often denoted as plus-one (+1), zero (0), and minus-one (−1). The plus-one and minus-one states are differentiated by a 180 degree optical phase shift. An optical duobinary data stream is typically created by driving a single Mach-Zehnder modulator (MZM) with a three level electrical drive. The modulator is biased at the null point in its transfer function and driven with an electrical signal that has three levels, where the upper and lower rails of the drive signal are separated by two times the required switching voltage of the modulator. This creates a three state optical output from the modulator where the upper and lower rails of the electrical drive signal produce a plus-one and minus-one state, respectively, and the middle state of the electrical drive signal creates a zero state from the modulator.
One technique for realizing an optical duobinary data stream is to create the required three-level electrical drive signal, used to drive the optical modulator, by filtering a differentially encoded binary NRZ data stream with a low pass electrical filter. This technique is referred to herein as the electrical low pass filtered (LPF) duobinary approach. An example of an optical transmission system using conventional electrical LPF duobinary is shown in FIG. 1.
Electrical LPF duobinary transmission uses an optical Mach-Zehnder modulator (MZM) biased at a null point in its transfer function and driven at about two times its required switching voltage (2*V). In such a configuration the required response bandwidth of the driver/filter/modulator combination to an input impulse is much lower than that needed for an NRZ transmitter. However, for optimal performance in, for example, a 10 Gb/s electrical LPF duobinary transmitter, a 3 GHz bandwidth first-order Gaussian low pass electrical filter is used as the ‘bandwidth bottleneck’ or bandwidth limiter. The bandwidth of the modulator and electrical driver in such a configuration are made to be significantly larger than the Gaussian electrical filter pass band in order to let the carefully designed filter create an appropriate spectral content for the data stream. Therefore, the 3 GHz electrical filter response dominates the transmitter response and is indicative of the preferred aggregate driver/filter/modulator response of the transmitter. Significant deviations from this ideal response, in bandwidth and/or response ripple, tend to seriously degrade the quality of the signal at the output of the transmitter. Therefore, great care is taken in achieving the proper transmitter response for use in a commercial system.
It is important to note that, within typical electrical LPF duobinary transmitter circuits, impedance matching must be maintained between various components within the circuit including the amplifier, electrical filter and modulator in order to minimize signal reflections. Reflections between the electrical filter and modulator, for example, can seriously degrade the back-to-back performance of a filtered duobinary transmitter. (Back-to-back performance of the duobinary transmitters as used herein, refers to the quality of the data stream at the output of the transmitter without transmission across a transmission link.) To avoid signal reflections great care must be taken to minimize impedance mismatches within the transmitter. Such stringent transmitter specifications tend to substantially increase transmitter component costs. Accordingly, relaxing these constraints on system architecture would significantly improve transmitter cost and yield.
It has been demonstrated that an approximate filtering function for realizing a duobinary data stream can be created within the response of a Mach-Zehnder electro-optic modulator (see Enning, “Signal Shaping For Optical Wideband Transmission Systems Using Inherent Lowpass Behavior of Counterpropagating Optical and Electrical Signals in a LiNbO3 Mach-Zehnder Modulator”, J. Opt. Commun. 22 (2001) 746 pp. 1-5, 2001). The Enning device employs an idealized magnitude sinc function response for the modulator. However, such a device response is not realistic for practical devices or for higher bit rate (e.g. ˜10 Gb/s) applications. The use of a modulator with a sinc magnitude response has a monotonic decrease in the quality of the data pattern with an increase in residual dispersion. Therefore, the sinc response modulator creates a data pattern that is not as robust against accumulated dispersion within a transmission link.
It has also been recognized that residual dispersion from transmission can improve the quality of an electrical LPF duobinary data stream. Specifically, it is understood that the optical spectral components that make up an electrical LPF duobinary data stream are not optimally aligned when initially transmitted; however, after propagation in standard single mode fiber (SSMF) dispersion can realign some of the spectral components within the bit stream such that the eye diagram improves, resulting in an improved optical signal to noise ratio requirement for a given bit error ratio. However, it has not previously been understood that the amount of improvement in the electrical LPF duobinary data stream produced by residual dispersion varies significantly with the initial quality of the bit pattern. That is, it has not been previously recognized that a lower quality duobinary data stream generated using a transmitter having a low bandwidth modulator can be dramatically improved with an appropriate amount of residual dispersion, and that a relatively high quality duobinary data stream shows significantly less improvement.
Accordingly, a need exists for an optical duobinary transmitter and method for optical duobinary transmission for higher bit rate applications, which are practical, relatively less technically complex and are cost effective.